Muscle injuries are one of the most common sports injuries, presenting an incidence of 10-55% of all injuries (Best and Hunter, 2000; Huard et al., 2002; Jarvinen et al., 2005). Delayed-onset muscle soreness (DOMS), an entity of ultrastructural muscle injury, is classified as an overexertion-functional muscle disorder type Ib according to the Munich Consensus Statement (Mueller-Wohlfahrt et al., 2013). DOMS is caused by high eccentric muscle contractions or unaccustomed exercises (Armstrong, 1984). Biopsy analysis of muscle has revealed that eccentric training causes Z-band streaming and broadening and destroys sarcomeres in the myofibrils (Friden et al., 1983), which leads to myofiber necrosis and inflammation (Paulsen et al., 2012).
DOMS is one of the most common reasons for impaired muscle performance in sports and is associated with muscle soreness, reduced muscle strength, and range of motion, and is frequently observed both in professional and recreational athletes (Kim and Kim, 2014; Mizuno et al., 2016; Pearcey et al., 2015). The signs and symptoms begin 6-12 h after exercise, increase progressively until they reach peak pain at 48-72 h, and decrease until they disappear 5-7 days later (Valle et al., 2013).
Lower limb compression garments are increasingly popular among athletes who wish to improve performance and to reduce exercise-induced discomfort and injury risk (Beliard et al., 2015). Furthermore, compression stockings have wide, evidence-based application for treating clinical pathologies such as deep vein thrombosis and chronic venous insufficiency (Ibegbuna et al., 2003; Scurr et al., 2001).
The impact of compression therapy during exercise or during recovery and its clinical outcome regarding DOMS is a subject of controversy (Beliard et al., 2015; Goto et al., 2017; Hill et al., 2014). Most results suggesting that compression garments are effective in enhancing recovery of muscle damage, whereas no clear relation between regeneration and applied pressure or wear time are known and underlying mechanisms are poorly understood (Beliard et al., 2015; Hill et al., 2014; Marques-Jimenez et al., 2016).
T2-weighted magnetic resonance images are frequently utilized for detecting muscle damage resulting from repeated eccentric muscle contraction (Nosaka and Clarkson, 1996; Valle et al., 2013; Yanagisawa et al., 2015). Increased T2-weighted signal intensity indicates intramuscular fluid accumulation, which shows good correlation to the degree of ultrastructural damage in the context of DOMS and suggests the use of MRI for the quantitative assessment of exercise-induced muscle damage (Nurenberg et al., 1992; Yanagisawa et al., 2015).
To our knowledge, no one has proven the somewhat speculative concept that applying compression attenuates changes in osmotic pressure and reduces venous blood pooling, the space available for swelling, hemorrhage, or hematoma formation after exercise-induced muscle damage (Duffield et al., 2014; Kraemer WJ, 2004). Hence, the purpose of this study was to investigate the influence of commercially available sport compression garments on the development of exercise-induced intramuscular edema in the context of DOMS. We hypothesized that wearing compression garments during recovery reduces intramuscular edema caused by disruption of the muscle fiber structure during eccentric training by reducing the available space for swelling.
The local ethics committee approved the performance of the present study with no requirements (Friedrich-Alexander-University (FAU) Erlangen-Nuremberg, Erlangen, Germany). All participants were informed of the benefits and risks of the investigation prior to signing an institutionally approved informed consent document to participate in the study.
Fifteen healthy volunteers (7 female; 8 male; age, 25 [+ or -] 6 years; height, 1.79 [+ or -] 0.11 m; body weight, 69 [+ or -] 12 kg) from the medical and sports faculties were recruited as participants. No participant had signs, symptoms, or history of chronic diseases and no current acute or overuse lower limb injuries or muscle injuries in their history. No participant had lower limb malalignment, and all participants presented full ranges of motion for the hip, knee, and ankle joints. The participants were advised to avoid any sports activities for 1 week prior to the investigation date. Exclusion criteria were any symptoms of lower limb muscle soreness 3 months prior to the study and regular training habits, including eccentric or plyometric exercises. All participants trained regularly in recreational sports, including swimming, triathlon, soccer, running, boxing, and lacrosse. The average training frequency of the participants corresponded to grade 3 of the Valderrabano Sport Scale (2.6 [+ or -] 0.4) with more than 5 training hours per week (Valderrabano et al., 2006).
Standardized eccentric exercise of the calf muscles
To induce DOMS, the participants performed a standardized, established eccentric calf muscle exercise (Hotfiel et al., 2017; Kellermann et al., 2017). During the whole exercise the participants were instructed and motivated by a licensed fitness coach.
Each participant warmed-up using two sets of heel raises of 15 repetitions, with one 20-sec break between the sets. Immediately after the warm-up, the eccentric exercise was performed on a specifically manufactured stair (Figure 1). It started with the participants raising their heels, maximally contracting the calf muscle (Figure 1A) for 1 sec and then putting the heels down slowly within 3 sec until the soles touched a -35[degrees] slant plate (Figure 1B). To return to the starting position (heels raised) and to relieve the calf muscle during concentric contraction, the participants pulled themselves up on a pull-up bar installed above their heads to focus on eccentric contraction (Figure 1C). To increase the load, each subject wore a weighted vest bearing approximately 25% of their body weight during the whole exercise. All participants performed 5 sets of 30 repetitions and rested 10 sec between each set, whereby the last set was performed until muscle fatigue, so that no further repetition of eccentric exercise was possible.
Immediately after standardized eccentric exercise, a conventional sports compression garment, compression class I (Compression Sock Run & Walk, 18-21 mmHg, 97% polyamide, 7% elastane, Bauerfeind AG, Zeulenroda-Triebes, Germany) was placed according to the manufacturer's instructions on one randomized calf (Figure 2). The compression sock was worn continuously for 60 h after eccentric exercise and was removed for the first time for follow-up examination.
Creatine kinase (CK) levels
Blood CK levels were measured at baseline and 60 h after the eccentric exercise. Approximately 5 mL blood was collected by vein puncture from an antecubital vein into serum tubes. CK measurement was conducted using the UV test according to the IFCC method (37[degrees]C) (Cobas 6000, Roche Diagnostics, Mannheim, Germany).
Muscle soreness assessment
The level of muscle soreness was evaluated using a pain scale (Mundipharma GmbH, Limburg, Germany) that displays a visual analogue scale (VAS) of 100 mm on the front (0, no pain; 100, worst pain) and a numerical rating scale (NRS) on the back. Muscle soreness was assessed at baseline and 60 h after the exercise. The participants were asked to mark their soreness at rest and during activity (going downstairs).
Range of motion (ROM)
The range of motion of both ankle joints was assessed manually with a goniometer. Representative was the maximal passive dorsiflexion at baseline and at post-intervention, while the participant was sitting off the edge of table with free hanging lower legs. The center of the goniometer was positioned on the lateral malleolus with one side horizontal next to the calf and the other side parallel to the lateral foot and being adjusted in accordance to the ability of motion. All measurements of the ROM were performed by the same orthopedic surgeon (T.H.) (Hotfiel et al., 2017; Kellermann et al., 2017).
The maximum calf circumference was measured using a tape measure while the participant was sitting off the edge of table with free hanging lower legs. The location of measurement was labeled with a permanent marker at baseline to ensure the same location was measured at follow-up. All measurements of the calf circumference were performed by the same orthopedic surgeon (T.H.) (Hotfiel et al., 2017; Kellermann et al., 2017).
Magnetic resonance imaging
MRI was performed at baseline and 60 h after standardized eccentric exercise. No MRI contraindications were stated for any participant.
Imaging of both lower legs was carried out simultaneously at the Department of Radiology of the University Hospital Erlangen (Germany) with a 3T MRI scanner (Magnetom Skyra Fit 3T, Siemens Healthineers, Erlangen, Germany) using a dedicated 32-channel spine coil and an 18-channel body coil (Siemens Healthineers).
An axial T2 mapping sequence (total acquisition time, 9:05 min; echo time, ranging from 10.64 to 190.8 ms; repetition time, 3000 ms; resolution, 0.7 x 0.7 x 8.0 mm), an axial T2-weighted turbo inversion recovery magnitude (TIRM) sequence (total acquisition time, 3:31 min; inversion time, 260 ms; echo time, 69 ms; repetition time, 5120 ms; flip angle, 145[degrees]; resolution, 0.8 x 0.8 x 4.0 mm), and a coronal T2-weighted TIRM sequence (total acquisition time, 3:42 min; inversion time, 260 ms; echo time, 68 ms; repetition time, 5120 ms; flip angle, 180[degrees]; resolution, 0.9 x 0.9 x 4.0 mm) were applied to the lower leg. Additionally, we performed a T1-weighted turbo spin-echo (TSE) sequence (total acquisition time, 3:01 min; echo time, 14 ms; repetition time, 582 ms; resolution, 0.7 x 0.7 x 5.0 mm) to depict the anatomy and morphology of the lower leg.